1. Field of the Invention
The present invention is related to transistor amplifiers having low gain and high frequency response or band width. The amplifiers produce a matching transitional voltage level characteristic for a family of transistors.
2. Description of the Prior Art
Prior Art amplifiers are known which are capable of achieving high gain amplification with low band width. To achieve higher frequency response or band width with such devices, feedback circuits have been employed. Aside from the extra components required to construct feedback circuits, such feedback circuits also produce signal phase shift which requires additional compensating circuits. Each element added in a feedback or compensating circuit also adds additional noise. When very high gain amplification devices are employed the devices create inherent d.c. problems which usually require adjustable or compensating circuits.
Amplifiers for optical transistors may be generally classified into three types for purposes of this explanation, namely (1) Discrete transistor amplifiers, (2) Operational amplifiers and (3) Darlington pairs of connected transistors.
Discrete transistor amplifiers are not available with gain characteristics below a factor of 20 to 1, accordingly, it is impossible to employ a discrete transistor amplifier between a photo transistor and a logic driver without employing feedback circuits which will reduce the gain.
Operational amplifiers are commercially available, however, such amplifiers have gain ratios in excess of those found in discrete transistor amplifiers. The usual amplification gain for operational amplifiers may vary from a low of around 400 to 1 to in excess of 100,000 to 1. Operational amplifiers require feedback circuitry to reduce the very high gain and in addition require some form of phase shift compensation to prevent oscillation. Most operational amplifiers generate an inherent d.c. level and require additional compensation elements to eliminate the d.c. offset.
Transistor amplifiers in Darlington Pair configurations have gain ratios in excess of the gain ratios achieved by discrete transistor amplifiers. Darlington Pair amplifiers are inherently slow because they have inherently high impedence characteristics. Darlington Pair amplifiers are slower yet when the first stage is a photo transistor and comprises part of the Darlington Pair. It has been found that Darlington Pair amplifiers having high impedance have low frequency response and are not suitable for use with high speed computer peripheral devices. Darlington Pair amplifiers are known which have low impedance and such amplifiers have higher frequency response, however, such low impedence Darlington Pair amplifiers do not have high enough voltage output to be compatible for driving a transistorized logic output device without using other discrete stage amplification. While the Darlington Pair amplifier does not require feedback amplification to reduce its gain, the last stage of amplification may require feedback to produce a proper transitional level for compatibility with the logic output driver.
In addition, the Darlington Pair amplifier has its own unique problem. The lowest voltage drop across the output (V.sub.ce) of a silicone transistor Darlington Pair is approximately 0.8 volts. A single stage bipolar transistor has an operational level below 0.4 volts, accordingly, such devices when connected to the output of a Darlington Pair configuration cannot be employed in computer logic circuits where dependability is a necessity. Stated differently, the voltage output, Vce of the first stage plus the base to emitter voltage (V.sub.be) of the second stage can be no lower than 0.8 volts, thus, it is the characteristic of the output of the second stage of a Darlington Pair amplifier to never be able to drop below 0.8 volts. It will be noted that the low voltage logic level for the input to a logic driver of the bipolar transistor type is specified as 0.8 volts. Accordingly, there is no more margin of operational assurance at the low level.
The greatest problem with the attempt to substitute fiber optic cables for coaxial cables has been the expense of the optical amplifying system and the compatible logic drivers.
The next greatest problem incurred in substituting fiber optic cables for coaxial cables has been the need to miniaturize the optical amplifying system and compatible logic drivers.
Another of the present limitations with being able to substitute fiber optic cables for coaxial cables is that the frequency response of the optical amplifying system is too low to permit use with high frequency response and wide band width systems.